Hello RRS1,
It is not a center port, it is a machining for the bolt that holds the secondary piston.
The bolt stem glide in the machining of the secondary piston.
If you have removed the bolt, without precaution, you have surely blocked the secondary piston, it is necessary to disassemble the master cylinder and reassembling being careful to put the bolt stem in the machining of the secondary piston
Below the internal parts of a brake master cylinder: